Various methods of providing documents, such as negotiable instruments, with tamper evident systems to prevent alteration have been proposed. Likewise, systems have also been developed to prevent reproduction of documents by photocopying to reduce the incidence of fraud. However, in recent years advanced color copiers which are readily accessible to the general public can produce nearly exact duplicates of the original document. It is very difficult for the untrained person to distinguish the original from an illicit reproduction. To prevent passing off of the reproduction as the original, efforts have been made to make the original document incapable of being copied or to incorporate authenticating systems into the document.
One known method of producing a paper that is useful for preventing fraud in security documents is to print the paper with a chemical in the form of an invisible image. When a bleach solution or an ink eradicator is applied over the area of the invisible image, a colored image appears to confirm its authenticity. However, this method is often undesirable because the bleach solution is corrosive, the ink eradicator contains toxic solvents, and the solution tends to warp or otherwise permanently damage the document.
Another method is to print a document with a fluorescent ink which is colorless under visible light, but becomes discernible when exposed to ultra-violet light. Similarly, a fluorescent ink may be printed on a document for automatic identification by an electronic scanner such as that disclosed in U.S. Pat. No. 4,186,020 to Wachtel. The use of fluorescent ink requires access to a source of ultraviolet light or a document verifying electronic scanner. This prevents quick and easy verification of the document.
A further prior method is disclosed in U.S. Pat. No. 4,183,989 to Tooth in which security paper is provided with a strip, thread or planchette having at least two machine verifiable security features, one of which is a magnetic material and a second of which is a luminescent material, an X-ray absorbent or a metal. Since most security documents are printed paper products with different base weight, color and texture requirements, the availability of various papers having these security features is very limited. This system also has the disadvantage of requiring specialized and expensive verifying machines to examine the documents and determine its authenticity.
U.S. Pat. No. 4,726,608 to Walton discloses the use of an opaque coating over an authenticating indicia. The image of the indicia is later made visible by scratching off the opaque coating or by applying a solvent to disperse the coating. This system further has the disadvantage of either requiring solvents or producing unwanted dust.
Still another method identifying forgeries is by micro-printing where a message, logo, or name is printed in very small type as a border around the face of the document. Generally, the message cannot be read by the naked eye, and requires some magnification to make it legible. Copiers cannot duplicate the micro-printing with sufficiently high resolution, so that on a copy the words become Just a broken line or blur. However, the tellers at a bank or persons accepting the document generally do not have access to magnifiers and thus, a copy will not be easily noticeable.
"Sunburst" or "Rainbow" printing is sometimes used to help defeat color copiers in which the document is printed with one intense color at one side of the document which slowly fades into another color across the width of the paper. Eventually the second color intensifies at the opposite side. This is a visual system that is difficult to duplicate on a copier. If the original printing is not available for comparison, it is extremely difficult for the untrained eye to know it is a copy or authentic.
Another method is disclosed in U.S. Pat. No. 4,210,346 to Mowry Jr. et al. in which "VOID" or other warning messages is pre-printed in halftone or multitone on the document to camouflage the pre-printing. These pre-printed messages are blended into the surroundings of the document so that human eyes see them as a uniform printing. The pre-printed "VOID" or warning messages will appear on the illegal copy by many color copiers. However, more sophisticated color copiers have been developed in recent years so that a skillful lawbreaker can now reproduce a security document without the preprinted warning messages appearing on the copies.
Another method of reducing the risk of fraud is to incorporate a color forming substance into the substrate of the document. To verify the authenticity of a document, a second color forming substance which is capable of taking part in a color-forming reaction is applied to the security document to reveal the hidden images or produce color changes. Examples of this form of detection system are disclosed in U.S. Pat. Nos. 4,037,007 to Wood and 4,360,548 to Skees et al. However, the second color forming substance is not generally available at every location where the security documents are handled or whenever it is needed. Thus, these documents can not be readily tested for their authenticity.
The above noted security and authentication systems have not entirely prevented unauthorized or fraudulent reproduction of documents. Accordingly, a need still exists for a simple and efficient system which is able to accurately authenticate a document and distinguish it from a reproduction.